The radar performance is expressed by a radar equation. To improve the radar performance, the following actions are generally taken in terms of parameters expressed in the radar equation: (a) increase of a transmission peak power and a pulse width; (b) increase of an antenna gain; (c) utilization of a long wavelength; (d) lowering of a system noise temperature; (e) reduction of a system loss; and the like.
The increase of a transmission peak power, the increase of the antenna size, and the like have a lot of restrictions and lead to an increase of the system size, and thus are under certain limitations in implementing these. The utilization of a long wavelength is difficult due to recent radio wave resource shortage.
Additionally there is an extremely strong demand for improving the radar performance, because of the advent of a so-called stealth, i.e., an object whose radio wave reflection is intentionally reduced. To meet the demand, highly sensitive reception performance is also needed, and thus the reduction of a system loss and the lowering of a system noise temperature are required.
Although the system loss includes various types of losses, a representative loss is a transmission feed loss occurring between a transmitter and an antenna. To reduce the transmission feed loss, used is an active array antenna using a number of modules called T/R modules (i.e. transceiver modules) each, as a single unit, having functions of transmission amplification, transmission-reception switching, and reception. In particular, the mainstream system is an active phased array system.
The active phased array system is an antenna system in which modules called the T/R modules (transceiver modules) are arranged. Between an antenna element and a phase shifter, each transceiver module incorporates: a transmission-reception switching function; and both (or either) of a transmission amplifier in a transmission system and a low noise amplifier (LNA) for reception in a reception system.
When the transmission amplifier is incorporated into the transceiver module, the module is arranged close to the antenna element. Thus, a feed loss due to a waveguide or the like does not occur, and the transmission feed loss can be limited to a loss from only essential components (such as a circulator).
When the reception LNA is incorporated into the transceiver module, a loss in reception can be reduced while multi-beams can be formed. That is, a received signal amplified by the LNA can be distributed into multiple signals without deteriorating S/N ratios, and thus multiple independent received beams can be formed from the distributed received signals.
The active phased array antenna which has the multiple received beams can detect different targets simultaneously and implement multiple functions simultaneously and independently. Thus, the active array antenna including at least the reception LNA provided for each antenna element is suitable to multi-functionalization requiring multi-beams.
Formation of the multiple received beams (received multi-beams) requires as many beam synthesis circuits as the received beams. Specifically, a signal received by each antenna element is amplified by the LNA and then is distributed into as many signals as necessary received beams, and the signals pass through attenuators, phase shifters, and the like which perform amplitude weighting for suppressing a side lobe and phase weighting for controlling the beam directivity. Thereafter, the beam synthesis circuits synthesize the received beams from the signals. The need for the multiple beam synthesis circuits causes a problem of increasing the system size.
Even the active phased array antenna cannot make the reception system highly sensitive by making the feed loss occurring between the antenna and the LNA close to zero and by reducing an internal noise of the LNA.